This proposal investigates the influence of serotonin (5-HT) and its associated neurons in the hypothalamic paraventricular nuclei (PVN) on neuronal corticotropin releasing factor (CRF) mRNA expression and feeding behavior in rats. PVN CRF neurons are important mediators in the control of feeding behavior; e.g. exogenous application of CRF or stimulation of CRF neurons result in the inhibition of food intake. Similarly, drugs that stimulate synaptic 5-HT transmission in the brain are also well known to inhibit food intake. Anatomically, it is also known that 5-HT terminal projections are found on PVN CRF neurons; this suggests that 5HT agents may suppress appetite through heretofore uncharacterized actions on PVN CRF neurons. A potential functional link between 5-HT is further supported by data demonstrating the presence of mRNAs for 5-HT receptor subtypes linked to appetite regulation in PVN CRF cell bodies. Remarkably, despite the suggestion of a number of studies, there have been no definitive studies to date demonstrating that 5-HT-associated drugs may indeed suppress feeding by stimulating CRF mRNA production and CRF protein expression. In this proposal, we hypothesize that there exists a functional link between serotonin (5-HT) and neuropeptides, such as CRF and melanocyte stimulating hormone (alpha MSH) and their respective neurons residing within the PVN (5-HT, CRF) and the arcuate (ARC) hypothalamic nuclei (alpha MSH), respectfully. Our Specific Aims are to (1) determine if 5-HT release regulates CRF or alpha MSH neurons in the PVN to suppress food intake; (2) Determine if 5-HT release indirectly regulates CRF neurons in the PVN via 1-MSH neurons residing in the arcuate and its associated neurons; and (3) determine downstream targets of hypothalamic CRF and its associated neurons. In our experimental design, we will utilize fenfluramine (FEN), a drug that strongly promotes 5HT synaptic transmission. Central and systemic FEN administration will be used to assess PVN CRF mRNA expression in rats pre-treated with 5-HT receptor antagonists specific to PVN CRF neurons that are appetite-linked (the 5HT-1B, 2A, & 2C subtypes). FEN will also be used to test whether 5-HT affects PVN CRF mRNA expression indirectly; 1-MSH release onto CRF neurons is stimulatory; hence, FEN could stimulate CRF neurons by increasing 1-MSH signaling. RNA interference will be used to eliminate CRF mRNA, to determine if FEN-initiated 5-HT influences appetite suppression by additional downstream mechanisms. These studies should provide new insight into the role of a poorly characterized 5HT-CRF synaptic connection in the control of food intake, and ultimately contribute to understanding if and how this connectivity is important to the regulation of overall energy balance, and the etiology of obesity, anorexia, and other appetite-related health problems. This proposal investigates the influence of serotonin (5-HT) and its associated neurons in the hypothalamic paraventricular nuclei (PVN) on neuronal corticotropin releasing factor (CRF) mRNA expression and feeding behavior in rats. These studies should provide new insight into the role of a poorly characterized 5HT-CRF synaptic connection in the control of food intake, and ultimately contribute to understanding if and how this connectivity is important to the regulation of overall energy balance, and the etiology of obesity, anorexia, and other appetite-related health problems.